1. Field of the Invention
The present invention relates generally to an electron gun for a cathode-ray tube, and particularly relates to a technique for improving high-frequency magnetic-field-transmittance characteristics of the electron gun.
2. Related Background Art
FIG. 14 is an enlarged cross-sectional view of a neck tube portion of a projection-type monochrome cathode-ray tube.
As shown in FIG. 14, magnetic field modulation is applied to an electron gun provided inside the neck tube 3 from outside the neck tube 3 by velocity modulation coils 20, to carry out the so-called velocity modulation of an electron beam so as to improve the focusing performance. This is the current advanced display technique (JP 10(1998)-74465 A). While an electron beam (not shown) is emitted from a cathode 7 and reaches a phosphor screen surface (not shown), the path of the electron beam is modulated by an alternate magnetic field generated by the velocity modulation coils 20, convergence yokes 23, deflection yokes 24, and the like.
The deflection yokes 24 are attached to a cathode-ray tube funnel cone portion, and generate an alternate magnetic field to deflect the electron beam path, so as to scan the cathode-ray tube phosphor screen surface with the electron beam. The convergence yokes 23 are attached to the outside the neck tube 3 of the cathode-ray tube, and generate an alternate magnetic field to deflect an electron beam path, so as to correct raster distortions and color shifts. The velocity modulation coils 20 are attached to the outside of the neck tube 3 of the cathode-ray tube, and generate an alternate magnetic field to modulate a scanning velocity of the electron beam, so as to prevent a high-brightness portion from extending off into a low-brightness portion on the phosphor screen surface, thereby obtaining sharp images.
A frequency of the alternate magnetic field for modulating an electron beam ranges from a deflection frequency (15.75 [kHz]) to a megahertz order equal to a level of picture frequencies. Therefore, there is a drawback in that the alternate magnetic field is attenuated by metal components of the electron gun that are formed by deep-drawing or the like with a metal material such as stainless steel, with the result that a desired electron beam modulation cannot be achieved.
As shown in FIG. 14, a part of the alternate magnetic field 19 generated by the deflection yokes 24 passes through a second anode electrode 11 (G5 electrode). An alternate magnetic field 22 generated by the convergence yokes 23 passes through the second anode electrode 11. The velocity modulation coils 22 are disposed between a first anode electrode 9 (G3 electrode) and a convergence electrode 10 (G4 electrode), and an alternate magnetic field 21 (at a level of 4 [MHz]) generated by the velocity modulation coils 20 passes through the first anode electrode 9 and the convergence electrode 10. When the alternate magnetic fields are applied to the electron beam via these metal electrodes, an eddy current is generated in part of the metal electrodes. Furthermore, as the frequencies of the alternate magnetic fields increase, loss of the alternate magnetic fields due to the eddy current increases. Therefore, the effect of the electron beam path modulation by the magnetic fields reduces in a high frequency range for the modulation.
A technique to solve such a problem is disclosed by JP 2000-188067 A. In the technique, an electron gun includes a G1 electrode that houses a cathode, a G2 electrode, a G3 electrode, a G4 electrode, and a G5 electrode that are arrayed in the stated order, the G3 electrode and the G4 electrode together forming a main electron lens. In the electron gun, a coil-shaped part is provided in a part of the G3 electrode so that a velocity modulation magnetic field passes therethrough, thereby reducing eddy current loss. Besides, the JP 61(1986)-29047 A discloses a technique of providing a plurality of slits on a circumferential surface portion of a cylindrical part of a bottomed cylindrical converging magnetic pole that is made of a nonmagnetic substance and that is attached to an electron-beam-emitting tip of an in-line-type electron gun, so as to prevent eddy current loss generated by a magnetic field that passes through the circumferential surface portion of the cylindrical part.
However, in the case where a technique of providing slits as taught by JP 61(1986)-29047 A is applied to an electron gun for forming an electron lens between bottomless cylindrical electrodes, the electrodes are deformed when slits are formed in cylindrical metal members. As a result, the electron lens formed between the electrodes has distortion, thereby being unable to achieve a desired effect of conversion of the electron beam having passed through the electron lens. Therefore, an electron beam spot observed on the phosphor surface has a deformed shape, thereby adversely affecting the resolution. Normally, in the case of cylindrical electrodes, when the electrode roundness thereof is lower than 99.8[%], the distortion of the electron lens apparently affects the electron beam spot. If slits are provided in the cylindrical electrode, the roundness of the electrode is impaired to 97 to 98%, thereby making the electrode unsuitable for practical application. In this case of a coil-shaped electrode taught by JP 2000-188067 A, it is not easy to form the coil so that it has a roundness of not less than 99.8[%], and in addition, a problem also arises in that the coil itself slightly hangs down due to the gravity. Therefore, it is impossible to avoid the problem of distortion of the electron beam spot shape caused by the deterioration of the roundness of the electrode.
The present invention is made to solve the foregoing problems, and it is an object of the present invention to provide a high-resolution electron gun for a cathode-ray tube with decreased resolution variation, which achieves a desired electron beam modulation effect without hindering the penetration of a modulation magnetic field from outside, and does not generate distortion of an electron lens due to distortion of an electrode, thereby obtaining an excellent beam spot shape.
An electron gun for a cathode-ray tube according to the present invention includes a plurality of cylindrical electrodes arranged so that an electron beam passes inside the electrodes, the electrodes being fixed to a support rod. In the electron gun, at least one of the electrodes is separated into at least two pieces along a plane substantially perpendicular to a central axis of the electrode, and a connecting member having a slit is provided between the pieces of the separated electrode. The pieces of the separated electrode are in electrical contact with each other via the connecting member, and are fixed to the support rod via the connecting member and support portions provided on a side of the connecting member.
The foregoing configuration reduces eddy current loss, since the modulation magnetic field passes through the slit provided in the connecting member. Furthermore, the electrode forming an electron lens at an end is configured so that the electrode is divided into at least two pieces and the connecting member with the slit is interposed between the two pieces. Therefore, it is possible to avoid a problem in that an electrode is deformed when the slit is formed in the electrode, and hence, it is possible to maintain a high roundness of an end surface of the electrode at which an electron lens is formed. Thus, by forming independently a portion of the electrode that is involved in forming an electron lens and the connecting member having the slit, and thereafter integrating the same, it is possible to prevent the distortion of the electron lens due to the deformation of an electrode forming the electron lens. According to the present invention, the roundness of an electrode forming a main electron lens exceeds 99.8[%], which is at the same level as that of an electrode without a slit. Therefore, an electron beam spot is not adversely affected by distortion of the electron lens.
The connecting member preferably is in a cylindrical shape whose cross-section taken along a plane perpendicular to the central axis is in a shape analogous to a shape of a cross section of the separated electrode.
According to the foregoing preferred configuration, the cross section of the separated electrode taken along a plane perpendicular to the central axis and that of the connecting member are analogous to each other. Therefore, it is possible to face and fix ends thereof with each other, or to insert one into the other and fix the same. Thus, the assembly of the electrode is facilitated.
The connecting member preferably includes two portions opposed to each other with a plane interposed therebetween, the plane being a plane including the central axis, or a plane parallel with the plane including the central axis.
Since the connecting member having the slit does not contribute to the formation of an electron lens, a high-precision forming process by deep-drawing, which results in the high production cost, is unnecessary. Therefore, by composing the connecting member of at least two portions with a plane including the central axis interposed therebetween, or with a plane parallel with the plane including the central axis interposed therebetween, as in the foregoing preferred configuration, it is possible to form the connecting member by a simple pressing process instead.
The support portions preferably are provided integrally with the two portions of the connecting member, respectively.
In an electrode formed by the conventional deep-drawing process, a support portion separately formed is fixed to a side of an electrode in a cylindrical shape by welding. In contrast, in the preferred configuration of the present invention, by composing the connecting member of two portions, it is possible to form the support portions and the connecting member by a simple pressing process so that they are provided integrally. This reduces the number of constituent components, thereby allowing for the lower-cost production. Furthermore, the welded portions conventionally cause the generation of foreign matters and the unnecessary discharge in the cathode-ray tube, but the number of welded portions can be decreased in the present invention. Furthermore, this reduces the cost.
The two portions of the connecting member preferably are coupled with each other by a coupling portion that is provided integrally with the connecting member at an end of the connecting member in a direction of the central axis.
According to the foregoing preferred configuration, it is possible to form the two portions of the connecting member as one component by a simple pressing process at a lower cost. Therefore, it is possible to decrease the number of the constituent components, and hence, to reduce the cost further.
The coupling portion preferably is provided integrally with the support portions at an end of each support portion in the central axis direction.
By providing the coupling portion integrally with the support portions, it is possible to form the connecting member in a cylindrical shape by only bending portions connecting the coupling portion with the support portions. Thus, the formation of the connecting member is facilitated.
It is preferable that an inner diameter of the cross section perpendicular to the central axis of the connecting member substantially is equal to an outer diameter of the electrode separated into two pieces, that ends of the two pieces of the separated electrode are inserted into ends of the connecting member in a direction of the central axis, respectively, and that the connecting member is conductive with the separated electrode.
According to the foregoing preferred configuration, the connecting member formed in an approximate cylindrical shape by bending is configured so as to catch the pieces of the electrode inserted into ends of the connecting member and fix the same. Therefore, the fixing of the foregoing members is facilitated. Furthermore, by providing the support portions in the connecting member, it is unnecessary to provide support portions on the pieces of the electrode at the ends of the connecting member.
It is preferable that the support portions include basal portions extending to ends of the two portions of the connecting member in a circumferential direction, respectively, and flat plate portions that are connected with the basal portions, respectively, and that are substantially parallel with the plane including the central axis. Besides, a relationship expressed as Dc less than 0.8 [mm] less than Ds preferably is satisfied, where Dc and Ds represent distances between the two support portions opposed to each other with the plane including the central axis interposed therebetween, Dc being a distance therebetween at a position where the basal portions are opposed to each other, and Ds being a distance therebetween at a position where the flat plate portions are opposed to each other.
In the case where the connecting member is composed of two portions and support portions are provided integrally with the two portions, a great distance Dc between the opposed support portions at the basal portions leads to the charging of the support rod as insulators. Then, an electric field generated therefrom passes through a gap at the basal portions thereby affecting the electron beam that passes through the inside. This causes the electron beam spot observed on the phosphor surface to be deformed in shape, thereby adversely affecting the resolution. This influence becomes remarkable when the distance at the basal portions exceeds 0.8 [mm]. On the other hand, the distance Ds between the opposed support portions at the flat plate portions preferably is greater than 0.8 [mm] since an excessively small distance Ds weakens the fixing of the connecting member to the support rod. In other words, by bending the basal portions of the opposed support portions in directions such that they approach each other so as to satisfy Dc less than 0.8 [mm] less than Ds, it is possible to obtain a configuration that is not likely to be influenced by variation of the potential caused by the charging of the support rod.
Furthermore, a method for manufacturing the above-described electron gun for a cathode-ray tube according to the present invention includes: punching and pressing a plate material to form the two portions of the connecting member, the coupling portion, and the support portions so that they are provided integrally, to form the slit in the connecting member, and to form the two portions of the connecting member in a predetermined curved shape; bending the plate material at borders between the support portions and the coupling portion so that each of angles formed between the support portions and the coupling portion is approximately 90xc2x0, so that the two portions of the connecting member are opposed each other to form a cylindrical shape; inserting ends of the two pieces of the separated electrode into ends of the connecting member on both sides in the central axis direction, respectively, so that the connecting member and the electrode are conductive with each other; and fixing the support portions to the support rod. The punching and pressing, the bending, the inserting, and the fixing are carried out in the stated order.
By the foregoing method, the processing of the connecting member and the assembly of the electrode are facilitated.